Conventionally, engine oil is being used for lubricating and cooling of an engine. The engine oil is stored in an oil pan provided under an engine. The engine oil circulates through each part of the engine by an oil pump. The engine oil circulating through each part of the engine falls to the oil pan. The engine oil falling into the oil pan circulates through each part of the engine again by the oil pump. During the process, the engine oil receives heat from each part of the engine and cools each part of the engine. The engine oil forms an oil film on each part of the engine, promotes lubrication between each of components, and restrains oxidation of the components.
Here, the condition of the engine oil is not adequate for the circulation through and lubrication of each part of the engine just after cold starting of the engine, because the engine oil stored in the oil pan has low temperature and high viscosity. It is therefore preferable that the temperature of the engine oil is raised as rapidly as possible and the viscosity of the engine oil is adequately adjusted, just after the cold starting. It is therefore already reviewed that: the oil pan is separated into sections; a condition where engine oil in one of the sections tends to be circulated just after a cold starting is made; and the engine oil in the section is promptly heated (with reference to Patent Document 1).
The prompt heating of the engine oil contributes to improvement of fuel efficiency caused by prompt reduction of friction. Recently, it is demanded that prompt heating of engine oil is further improved, because the improvement of fuel efficiency is strongly demanded. An oil pan structure of Patent Document 2 is suggested in view of this point. Patent Document 2 suggests a structure in which an oil pan includes an oil pan separator separating the oil pan into a main chamber having a suction port of the engine oil and a sub chamber not having the suction port, and a communication hole making a communication between the main chamber and the sub chamber is provided. The communication hole controls conduction of the engine oil between the main chamber and the sub chamber using viscosity changing of the engine oil. The main chamber may be called “a first chamber”. The sub chamber may be called “a second chamber”. The oil pan separator may be called “a sub oil pan”. The oil pan may be called “a main oil pan”.    Patent Document 1: Japanese Patent Application Publication No. 2001-152825    Patent Document 2: Japanese Patent Application Publication No. 2003-222012